Combustor and exhaust nozzle components on military and future hypersonic commercial engines require cooling to maintain the maximum component operating temperatures within the capabilities of metal structures. The requirement for cooling air reduces the engine performance. Ceramic materials, such as monolithic ceramics, offer potential for operation at the extreme temperatures these components would experience without cooling air. However, designers have had difficulties in the past utilizing monolithic ceramics in structural components due to low toughness and susceptibility to brittle fracture. Therefore, alternatives such as ceramic matrix composites and carbon matrix composites ("CMCs") have been developed to provide the toughness capabilities not achieved in monolithic ceramics.
Ceramic and carbon matrix composites are being actively pursued for structural applications in turbine engine nozzles. Typically, the composites are in the form of a laminate having a plurality of laminar layers joined by a matrix material. Each laminar layer has a plurality of fibers that provide the layer with strength. However, a major drawback to the application of these composites has been their weak inter-laminar strength (i.e., any strengths which are dependent on the matrix as opposed to the fibers). The fibers are typically several orders of magnitude stronger than the matrix, therefore, the preferred designs are those which minimize the stresses in the matrix. The lack of a high temperature fastener has complicated CMC's designs due to the requirement that the attachments occur in cooler regions where metal fasteners could survive.
One structural application in which composites have been pursued is that of a fastener. Composite fasteners such as composite bolts are attractive because they may be disposed in hotter regions than metal fasteners. A shortcoming in prior art composite bolts is the limited ability of the bolt to transfer load from the bolt head to the shank of the bolt where the load must be transmitted by shear. Another shortcoming of threaded composite bolts of the prior art is that the load is transferred from the nut to the bolt shank, again through shear in the threads. This shear load results in significant matrix or inter-laminar shear in the composite bolts and causes another weak link in CMC composites. Thus, although CMC materials can be threaded, the low inter-laminar strength of the materials precludes using threaded CMC fasteners in structural applications.
Accordingly, scientists and engineers working under the direction of Applicant's assignee have been working to provide a composite fastener for use in high temperature environments that can be used in combination with CMC's in structural applications.